A standard tubing extruder has a nozzle forming an annular and normally circular outlet passage. The inner wall of this passage is defined by a normally solid mandrel and the outer wall is defined at least at its downstream end by an adjustment sleeve. This sleeve is radially deformable or shiftable and is braced by a plurality of radially extending and angularly equispaced adjustment screws against a massive outside holding ring. Thus it is possible by advancing and retracting the screws to at least limitedly deform or shift the adjustment sleeve and, therefore, change the radial dimension of the extruder outlet.
In a standard system the finished product is subject to periodic measurements and the screws on the extruder are manually advanced or retracted, as necessary, to eliminate thick or thin spots in the extruded tubing and, therefore, make the product more uniform in wall thickness. Clearly such a system is labor intensive. In addition there is a substantial lag time between ascertaining that a correction is needed, making the correction, and having the correction show up in the finished product. In the meanwhile a large quantity of subtolerance tubing is produced.
It has been suggested in German utility models 8,526,188 and 8,701,877 as well as in German patent documents 2,113,054 and 3,512,097 respectively of K. Schwarze and M. Loozenski to provide an extruder head with a sleeve having a plurality of angularly spaced actuators taking the place of the prior-art screws. Similarly German utility model 8,704,664 shows an arrangement having a complex fitting replacing the downstream end of the extruder. All these systems require that the extruder be largely rebuilt in order to eliminate the manual part of the adjustment operation. Other systems such as described in German patent document 3,304,865 of E. Schwab et al rely on more complex control arrangements also not adaptable to existing equipment.